10 - Angrites

Summary

Introduction

Angrites are a group of basaltic achondrites with unusual mineralogy and ancient crystallization ages. They are ultramafic crustal rocks that have experienced no brecciation and little thermal or shock metamorphism [10.1]. According to the Meteoritical Bulletin, as of June 2014, and not accounting for pairing, there were 22 angrites; the type specimen, Angra dos Reis (stone), is the only fall (www. lpi.usra.edu/meteor/metbull.php). Angrites are divided into two subgroups on the basis of textural features related to petrogenesis: (1) coarse-grained, generally cumulate angrites that appear to have cooled slowly (‘plutonic’ angrites) and (2) finer-grained basalts that show evidence of rapid cooling (‘quenched’ angrites).

Mineralogy

Mittlefehldt et al. [10.1] reviewed the mineralogy and petrology of angrites; the following sections draw on the descriptions and data from this work and other studies [10.2–10.4]. Angrites are igneous rocks characterized by subequal abundances of olivine, pyroxene and plagioclase. The compositions of the major mineral phases are unusual. Olivine is calcium-bearing, and frequently contains inclusions or exsolution lamellae of kirschteinite. Orthopyroxene is absent, and clinopyroxene is a highly aluminian-titanian diopside (fassaite). Plagioclase is calcic and can be accompanied by celsian (a barium-containing feldspar). A calculated normative mineralogy for angrites is 13–55% olivine, 6–46% clinopyroxene and 25–39% plagioclase [10.3]. Accessory minerals include spinels (mainly hercynite and magnetiteulvöspinel), whitlockite, baddeleyite, troilite and iron-nickel metal. One angrite, D’Orbigny, contains abundant glass, filling open spaces, forming pockets and occurring as inclusions in olivine [10.5].

Compositional variations in mineralogy are related to the petrogenesis of the specimen: the plutonic angrites tend to contain relatively homogeneous crystals, whereas the quenched angrites have more strongly zoned grains, many of which contain exsolution lamellae. The compositional variation of pyroxene and olivine in four angrites is shown in Figure 10.1. Note the tight compositional grouping in both olivine and pyroxene in the plutonic angrites Angra dos Reis (AdoR) and LEW 86010. This contrasts with the spread in values for the fine-grained angrites.

Olivine is variable in both abundance (20–65 vol.%) and composition (Fa_5–95) across the angrites (Figure 10.1). NWA 1670 has the most magnesian olivine (Fa₄) [10.8, 10.9].

As well as variation in composition between samples, there is also significant core-to-rim variation in angrite olivines. For example, in A 881371, a quenched angrite, olivine trends from Fa_10.8 (core) to Fa_65.5 (rim) [10.7].